This invention relates to a method of and an apparatus for producing texture topograms at surface layers of non-amorphous polycrystalline bodies.
In a polycrystalline solid body, generally the crystallites are present statistically disordered.
When on the other hand such a body is subjected to certain strains, for instance a mechanical-plastical deformation, a cooling or the like, this has the result that the crystallites previously present in the body and completely statistically distributed in their orientations are now oriented prevailingly similarly in specific directions of preference. This more or less orderly orientation of the crystallites is generally called "texture". The textures influence the technological, such as the mechanical, electrical, magnetical or the like properties of the respective polycrystalline body in the various strain directions.
For the determination of topographies, there are well-known methods, which, however, are only suited for use in connection with monocrystals. On the other hand, methods of measuring textures are also known with which it up to now, however, is not possible to produce locally differentiating texture topographies.
Presently known topographies, such as X-ray topographies, are always only produced with monocrystals and serve the purpose of reproducing interferences, such as displacements, small-angle grain borders, twins and other domains. Topography methods working strictly monochromatically with a well-defined reproduction, such as according to Lang or Berg-Barrett, in doing so use the crystal to be examined itself as monochromator. However, the angle divergence of the radiation beam to be used is permitted to be only very small, i.e. in the range of angle minutes, and this again causes an only inferior utilization of the entire radiation resulting in the radiation emitter and also long exposure times (in part many hours up to days).
Upon employment of so-called count-tube methods, for an X-ray reflex (i.e. for a reflecting lattice plane family) in a pole figure the frequency of the different layers of reflecting lattice planes within the texture preparation may be determined. In this method, however, the determination is always effected throughout a larger area of the body to be examined, and no determination is obtained on the local distribution of the crystallite orientation, i.e. on the anisotropy of the texture.
The topography method according to Lang known since long ago among the topography methods today known surely has the best resolution and the maximum contrast. The resolution suffices in order to make visible individual displacements, and it is also possible to obtain a spatial impression on the position of the displacements stereoscopically by means of two exposures at different angles of view. In the method of Lang, an X-ray beam is used which prior to its impingement upon the monocrystal to be examined passes through an aperture. The monocrystal is then adjusted in view of the beam in such a way that the wavelength of the K.alpha..sub.1 radiation impinges with the Bragg angle upon the lattice plane family of the surface, the aperture in front of the monocrystal having to be provided in such a way that the angle of divergence of the radiation beam passing through the aperture is smaller than the difference to the Bragg angle of the wavelength K.alpha..sub.2 which at only a slightly larger diffraction angle would furnish the same topogram, and this would have to lead to undesired superimposition. The radiation diffracted at the reflecting lattice plane family then passes through a further aperture and is exposed on a film arranged therebehind parallel to the monocrystal. In the Lang topogram, disturbed regions such as for instance displacements appear more intensely blackened than the exposure of the nondisturbed environment thereof. In the topography of Lang, only a relatively small space angle element of the emitted radiation is able to be utilized, which along with an inferior utilization of the primary beam at reflexion results in substantial exposure times in part lasting days. The use of rotary anode X-ray tubes of a high thermal load capacity does provide a shortening of the exposure time, but frequently causes difficulties with local stability of the focal spot on the rotating anode. With the Lang method, the forming of an image of texture topograms of polycrystalline bodies also is not possible with reasonable exposure times.
In contradistinction to the Lang method wherein only the K.alpha..sub.1 radiation is used for forming an image, which thus represents a strictly monochromatic method, in the method of Berg-Barrett generally this separation is dispensed with, a certain inferiority of well-definedness awarely being accepted. Like all partially monochromatical methods, the image contains a substrate blackening which is caused by the diffraction of the retarding continuum. The Berg-Barrett method works with a simple reflexion method: The radiation of a certain wavelength K.alpha. emitting from the line focus of the X-ray tube is reflected upon a film, provided it is incident upon the monocrystal to be examined respectively at the Bragg angle. The image in the diffraction plane is well-defined and length-true, but in the plane normal thereof, an enlargement is effected, the overall image thereby being distorted. A condition for a well-defined image there is the use of only one wavelength, the monochromatisation being achieved at the monocrystal examined itself--similar to the method of Lang. The Berg-Barrett method requires a comparatively only low apparatus expense and also has shortened exposure times as compared with the method of Lang.
Focussing X-ray methods are also known (e.g. those of Seemann and Bohlin), wherein in setting out from a divergent beam of X-rays the origine of which is on the same circle as the crystal powder preparation to be examined the diffracted radiation is focussed back to a point of this circle. When a diffraction arrangement of Seemann-Bohlin is combined with a curved monochromator (e.g. Johansson-Monochromator), thereby the X-ray diffraction method of Guinier is made possible. This method is characterized in that it is strictly monochromatical and focussing. The strict monochromasy permits long exposure times and thus next to the measurement of strong also that of very weak intensities upon suppression of retarding radiation and interfering lines from the own spectrum of the tube.
In order to determine the texture of a polycrystalline body, texture measurement methods have become known which set out from a focussing according to Bragg-Brentano. This measurement methods result in the recording of a pole figure from which predominant textures are able to be noted, but in which a locally differentiated topography of the body to be examined is not perceivable.